CSC461%20Lecture%209:%20GLUT%20Callbacks

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CSC461 Lecture 9GLUT Callbacks

• Objectives
• Introduce double buffering for smooth animations
• Programming event input with GLUT

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GLUT callbacks

• GLUT recognizes a subset of the events recognized
  by any particular window system (Windows, X,
  Macintosh)
• glutDisplayFunc
• glutMouseFunc
• glutReshapeFunc
• glutKeyFunc
• glutIdleFunc
• glutMotionFunc, glutPassiveMotionFunc

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Double Buffering

• Instead of one color buffer, we use two
• Front Buffer displayed but not written to
• Back Buffer written to but not altered
• Program then requests a double buffer in main.c
• glutInitDisplayMode(GL_RGB GL_DOUBLE)
• At the end of the display callback buffers are
  swapped

void mydisplay() glClear() . / draw
graphics here / . glutSwapBuffers()
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Using the idle callback

• The idle callback is executed whenever there are
  no events in the event queue
• glutIdleFunc(myidle)
• Useful for animations

void myidle() / change something / t
dt glutPostRedisplay() Void mydisplay()
glClear() / draw something that depends on
t / glutSwapBuffers()
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Rotating Square
void display() glClear(GL_COLOR_BUFFER_BIT)
glBegin(GL_POLYGON) thetartheta/((23.14
159)/360.0) glVertex2f(cos(thetar),
sin(thetar)) glVertex2f(-sin(thetar),
cos(thetar)) glVertex2f(-cos(thetar),-sin(thetar
)) glVertex2f(sin(thetar), -cos(thetar))
glEnd()
void idle() theta2 if
(thetagt360.0) theta-360.0
glutPostRedisplay()
Specify the display and idle callbacks in main.c
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Using globals

• The form of all GLUT callbacks is fixed
• void mydisplay()
• void mymouse(GLint button, GLint state, GLint x,
  GLint y)
• Must use globals to pass information to callbacks

float t /global / void mydisplay() / draw
something that depends on t
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The mouse callback

• glutMouseFunc(mymouse)
• void mymouse(GLint button, GLint state, GLint x,
  GLint y)
• Returns
• which button (GLUT_LEFT_BUTTON,
  GLUT_MIDDLE_BUTTON, GLUT_RIGHT_BUTTON) caused
  event
• state of that button (GL_UP, GLUT_DOWN)
• Position in window

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Positioning

• The position in the screen window is usually
  measured in pixels with the origin at the
  top-left corner
• Consequence of refresh done from top to bottom
• OpenGL uses a world coordinate system with origin
  at the bottom left
• Must invert y coordinate returned by callback by
  height of window
• y h y

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Obtaining the window size

• To invert the y position we need the window
  height
• Height can change during program execution
• Track with a global variable
• New height returned to reshape callback that we
  will look at in detail soon
• Can also use enquiry functions
• glGetIntv(State_comp, ptr)
• glGetFloatv(State_com, ptr)
• to obtain any value that is part of the state
• State_comp could be GL_MODELVIEW_MATRIX,
  GL_PROJECTION_MATRIX, GL_VIEWPORT, etc.

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Window Management

• Multiple windows and subwindows
• Creating a window returns a window id
• GLInt id
• glutCreateWindow(A New Window)
• Change the current window
• glutSetWindow(GLInt id)
• Operations performed on the current window

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Terminating a program

• In our original programs, there was no way to
  terminate them through OpenGL
• We can use the simple mouse callback

void mouse(int btn, int state, int x, int y)
if(btnGLUT_RIGHT_BUTTON stateGLUT_DOWN)
exit(0)
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Using the mouse position

• In the next example, we draw a small square at
  the location of the mouse each time the left
  mouse button is clicked
• This example does not use the display callback
  but one is required by GLUT We can use the empty
  display callback function
• mydisplay()

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Drawing squares at cursor location

• void myMouse(int btn, int state, int x, int y)
• if(btnGLUT_RIGHT_BUTTON stateGLUT_DOWN)
• exit(0)
• if(btnGLUT_LEFT_BUTTON stateGLUT_DOWN)
• drawSquare(x, y)
• void drawSquare(int x, int y)
• yw-y / invert y position /
• glColor3ub( (char) rand()256, (char) rand
  )256, (char) rand()256) / a random color /
• glBegin(GL_POLYGON)
• glVertex2f(xsize, ysize)
• glVertex2f(x-size, ysize)
• glVertex2f(x-size, y-size)
• glVertex2f(xsize, y-size)
• glEnd()

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Using the motion callback

• We can draw squares (or anything else)
  continuously as long as a mouse button is
  depressed by using the motion callback
• glutMotionFunc(drawSquare)
• We can draw squares without depressing a button
  using the passive motion callback
• glutPassiveMotionFunc(drawSquare)

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Using the keyboard

• Register function
• glutKeyboardFunc(mykey)
• Callback
• void mykey(unsigned char key,int x, int y)
• Passed in ASCII code of the key depressed and
  mouse location
• Note GLUT does not recognize key release as an
  event

void mykey() if(key Q key q)
exit(0)
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Special and Modifier Keys

• GLUT defines the special keys in glut.h
• Function key 1 GLUT_KEY_F1
• Up arrow key GLUT_KEY_UP
• if(key GLUT_KEY_F1
• Can also check of one of the modifiers
• GLUT_ACTIVE_SHIFT
• GLUT_ACTIVE_CTRL
• GLUT_ACTIVE_ALT
• is depressed by
• glutGetModifiers()
• Allows emulation of three-button mouse with one-
  or two-button mice

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Reshaping the window

• We can reshape and resize the OpenGL display
  window by pulling the corner of the window
• What happens to the display?
• Must redraw from application
• Two possibilities
• Display part of world
• Display whole world but force to fit in new
  window
• Can alter aspect ratio

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Reshape possiblities
original
reshaped
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The Reshape callback

• Register function
• glutReshapeFunc(myreshape)
• Callback
• void myreshape(int w, int h)
• Passed in width and height of new window (in
  pixels)
• A redisplay is posted automatically at end of
  execution of the callback
• GLUT has a default reshape callback but you
  probably want to define your own
• The reshape callback is good place to put camera
  functions because it is invoked when the window
  is first opened

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Example Reshape

• This reshape preserves shapes by making the
  viewport and world window have the same aspect
  ratio

void myReshape(int w, int h) glViewport(0,
0, w, h) glMatrixMode(GL_PROJECTION) /
switch matrix mode / glLoadIdentity()
if (w lt h) gluOrtho2D(-2.0, 2.0,
-2.0(GLfloat)h/(GLfloat)w,
2.0(GLfloat)h/(GLfloat)w) else
gluOrtho2D(-2.0(GLfloat)w/(GLfloat)h,
2.0(GLfloat)w/(GLfloat)h, -2.0, 2.0)
glMatrixMode(GL_MODELVIEW) / return to
modelview mode /